Oscar Pistorius acted "hastily" and used "excessive force" which a reasonable person would not, said the judge who is deciding whether to convict the star of culpable homicide.
In a damning judgement, Judge Thokozile Masipa said: "I'm not certain a reasonable person would have fired four shots in to that small cubicle. A reasonable person would see a person... might die as a result."
An emotional Pistorius listened to the lengthy judgement with his relations behind him, alongside relatives of Steenkamp.
Masipa employed a "reasonable man" test to decide if Pistorius acted that night in a way that any 'reasonable' South African would have done in the same situation.
She said Pistorius had failed to take steps to avoid the death of Steenkamp and had acted "too hastily" and used "excessive force."
The court also heard that Pistorius could have summoned security if he had "picked up his cell phone" or "ran to the balcony and screamed," instead of opening fire like he did.
Masipa pointed out there was "no reason" why Pistorius could not have done that.
"It probably would have taken as much time - if not less, than to go to the bathroom and fire those shots," she said.
The critical ruling by the judge gave the impression that Pistorius is likely to be found culpable to some degree, which opens up the possibility of a custodial sentence for him.
But Masipa adjourned the hearing before announcing her verdict, meaning Pistorius is likely to find out tomorrow (Friday) if he faces up to 15 years in jail for the charge.
Pistorius earlier wept in court the judge found the fallen super star innocent of murder.
He was laid open to culpable homicide by being found not guilty of murder. The judge must rule how negligent Pistorius was on a sliding scale - with the jail term rising with the level of negligence.
Pistorius' defence team claimed his disability affected his judgement on the fateful night and therefore his action passed the "reasonable man" test.
Masipa said many South Africans were victims of violent crime, but that they did not all "sleep with firearms under their pillows."